Frequency synthesizers are used in a wide range of electronic equipment, particularly in radio and television tuners, cable modems and other high-frequency data communication equipment, and test equipment, therefore in many of these applications a wide frequency output range with low noise is desired and, in the past, it has been common to achieve these ends using a circuit based on a phase-locked loop (PLL) control system.
A phase-locked loop is a closed-loop feedback control system that generates an output signal in relation to the frequency and phase of an input reference signal, automatically raising or lowering the frequency of a voltage controlled oscillator until it matches the reference signal in both frequency and phase. This technique is widely used in radio, telecommunications, computers and other electronic applications and facilitates the stablization of a generated signal or the detection of signals in the presence of noise. The technique of constructing an integrated circuit to hold a complete phase-locked loop building block is well known in the art and is widely used in modern electronic devices, with signal frequencies from a fraction of a cycle per second up to many gigahertz.
The PLL circuits discussed herein are constructed using analog circuitry. The most common form of analog PLL is constructed from a phase detector, a low-pass filter and a voltage-controlled oscillator (VCO) placed in a negative feedback configuration. The PLLs discussed herein also contain frequency dividers in the feedback path and in the reference path in order to make the PLL output frequency an integer multiple of the reference signal frequency. Under initial conditions, a first assumption is that the VCO is generating an output signal that is likely to be at nearly the same frequency as the reference signal. If the phase from the VCO output signal falls behind that of the reference signal, a charge pump within the phase detector changes the control voltage output from the phase detector so that the VCO frequency increases. Likewise, if the VCO output signal phase creeps ahead of the reference, the phase detector causes the charge pump to change the control voltage to slow down the VCO. The low-pass filter serves the function of smoothing out the abrupt changes in control voltage from the charge pump. Since initially the VCO output signal may be far from the reference frequency, practical phase detectors may also respond to frequency differences, so as to increase the lock-in range of allowable inputs.
A well known method of achieving a wide frequency range for either single or multiple frequency bands up to and even beyond 2000 MHz is to have a separate voltage-controlled oscillator (VCO) working with the PLL for each frequency band. This technique requires significantly more components than a single-VCO circuit and adds more sources of noise to the circuit.
One method for addressing the problem of a wide frequency range while maintaining low noise is taught by U.S. Pat. No. 5,796,311 entitled PHASE-LOCKED LOOP CIRCUIT. Multiple pre-scaler circuits are used, each dividing the output signal frequency by a predetermined number. These separate pre-scaled signals are then combined back together to feed into the phase comparator of the PLL as a single signal. This approach adds considerable complexity to the circuit as multiple pre-scaling circuits are required.
Another method of providing a wide frequency range is taught by U.S. Pat. No. 6,788,157 entitled PROGRAMMABLE FREQUENCY SYNTHESIZER which discloses the use of a regenerative frequency divider and programmable integer divider to provide wideband frequency coverage from a single narrowband oscillator. However, the noise reduction provided by this circuit is quite limited, with no more than 3 dB of noise reduction realized by each of the three frequency divider elements disclosed in the invention.
Still another method of providing a wide frequency range along with improved noise performance is taught by U.S. Pat. No. 6,977,556 entitled RATIONAL FREQUENCY SYNTHESIZERS which discloses the use of multiple single side band (SSB) mixers for a range of frequency ratios based on the output frequency and the reference frequency. While this invention enables performance across a wide frequency range, the use of multiple SSB mixers, ratio dividers and associated control circuitry is a complex approach to providing a wide frequency range with low noise.